Pictures and video taken by concerned passersby show smoke billowing from block D of the building, which is owned by the City Council.
The fire department mobilised over 50 firefighters from Southsea, Cosham, Portchester, Havant, Eastleigh, Fareham, Hayling Island, St Mary’s, Waterlooville, Beaulieu and Alton to attend the scene.
The fire was ablaze in the fourth storey of the residential building, burning up the roof and the top floor of the flat on the level below.
The flat was not occupied, fire crews confirmed, with the block successfully evacuating all residents.
‘My mum was upstairs and I heard them (police) say there was a fire in our block,’ Portsmouth News quoted a 15-year-old girl as saying.
‘I was petrified. I was a bit panicky. I don’t know if my guinea pig Custard will be OK.’
62-year-old Block D resident Elaine Rowland said: ‘I’m anxious now. It’s something we’ve been worried about happening.’
A spokeswoman for the fire service said at 6pm that crews had managed to contain the fire, adding the incident was ‘expected to be scaled down shortly.’
The fire resulted in a partial road closure at the M275 slip road and full road closure of Mile End Road.